Use of mono or polyphase inverters, supplied from a source of direct or rectified current, to supply an alternating current to an electrical machine such as a synchronous motor for example, so as to be able to vary its speed, is known. In general, these inverters include electronic switches, such as insulated gate bipolar transistors (IGBT), which are fitted in a bridge and make it possible to connect an output of the inverter alternately to the positive and negative poles of the power supply. These inverters are in general controlled by a rectangular signal of fixed frequency (cutoff frequency), the pulse width of which is modulated to recreate a signal of quasi-sinusoidal form at the output of the inverter.
It is often advantageous to use several inverters in parallel, to limit the amperage of the currents produced by each inverter (and therefore the size and cost of the components which constitute it), and/or for purposes of redundancy, or even, thanks to a so-called interlaced control, in which the control signals of the different inverters are offset relative to each other, to limit the disturbances which are applied to the source of direct current at the time of the current surge produced by simultaneous switching of the transistors.
From the document U.S. Pat. No. 7,109,681 for example, a device for controlling a conventional synchronous motor from two three-phase inverters producing a signal in phase with the fundamental frequency is known. The inverters are controlled by chopped control signals phase-shifted by 180° relative to each other. This phase shift causes disturbances which are superimposed on the power supply signal of the motor at the fundamental frequency, and are filtered in the resulting signal by the combination of the signals of the two inverters in the interphase transformers of which, for each phase, the ends are connected respectively to each output of the inverters, and the midpoint is connected to the motor.
However, it is sometimes necessary, e.g. because of questions of redundancy at the level of the electrical machine itself, to use a machine formed out of several connected sub-machines, such as, in the example of a synchronous motor, a motor including multiple independent stator windings mounted in a star. These motors are usually called n-star motors, where n corresponds to the number of distinct stator windings.
When they associated an inverter with each sub-machine and controlled the inverters by an interlaced control, the inventors noticed that the known disturbances of the alternating control signal of the motor, which were known and attributed to instantaneous circulation of currents between the outputs of the inverters, which were connected galvanically in the prior art, were also present in this assembly, where the inverters are insulated galvanically from each other.
It was also noticed that these disturbances caused disturbing currents in the motor, generating heat and losses of yield by Joule effect, Foucault currents and hysteresis, for example.